The hypothesis of this proposal is that arrhythmic and non-arrhythmic sudden death can be differentiated in dogs using a combination of right ventricular (RV) pulse pressure (delta P), mixed venous oxygen saturation (RVS02), heart rate (HR), and repolarization abnormalities in acute and chronic models, and both mechanisms of sudden death can be differentiated from normal activities in dogs with normal and reduced left ventricular function. RV SO2, RV delta P and HR will be recorded by a single transvenous lead incorporating an oxygen saturation sensor and a pressure sensor placed in the RV. Repolarization indices (QT interval, the activation-recovery interval and the QRST integral) will be recorded using a multilead body surface electrocardiographic recording system in the acute experiments, and using an implanted subcutaneous 3 electrode recording system in th chronic experiments. Acute experiments will be performed to 1) measure the change in hemodynamic status and repolarization abnormalities prior to 5 mechanisms of sudden death in normal dogs and 2) develop criteria for accurate distinction between arrhythmic and non-arrhythmic sudden death on the basis of HR, RV DO2, RV delta P and repolarization measurements. Experiments in chronic dogs with normal and abnormal ventricular function will be performed to 1) determine the normal variation in RV DO2, delta P and repolarization measurements in dogs with normal and abnormal ventricular function, 2) assess the effect of exercise on RV SO2, delta P and repolarization measurements, 3) assess the stability and accuracy of the chronic recordings and their ability to distinguish mechanisms of sudden death in permanently implanted systems and 4) define a detection algorithm that can be incorporated into an implantable monitor to detect arrhythmic and non-arrhythmic death. If an implantable monitor can be shown to differentiate arrhythmic from non-arrhythmic sudden death, additional funding will be sought t perform a study in humans. The long term goal is to implant a monitor device in humans at risk of sudden death to determine the mechanisms underlying sudden death. The information obtained may allow identification of patients at risk for true arrhythmic death, and more appropriate direction of treatment for the prevention of sudden death.